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Cementation Process.



composition, it may be made either from wrought or cast iron.
We shall first consider the former method.

Cementation.—In this, the oldest process, bars of wrought
iron are placed in fire-clay boxes, Pig. 88, with charcoal dust
around and between them, and a layer of fire-clay over all
(being the cement giving the name to the process). They are then
subjected to a bright-red heat, for a time varying with the amount
of carbon required to be introduced, and which may be as much
as a fortnight for the more highly carbonised steels. The charcoal
then becomes combined with the iron, and the steel so pro-
duced is called blister steel, from the fact that the bars are covered
with blisters. These bars are next broken up, piled, and heated
in a furnace almost exactly like the one in Fig. 85, hammered by
rapid blows from a tilt-hammer, Fig. 89, and shear steel of a

fibrous quality is thus produced.* Double shear steel is made by
breaking in two and re-hammering Crucible cast steel is obtained
by melting fragments of blister steel in covered crucibles made of
a mixture of fire-clay and plumbago, and placed in sets of six or
twelve in furnaces having a similar section to the one shewn in

* The steam hammer is used in later-built works.      For drawing,  see
Chapter IV.